From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-136.mta0.migadu.com [91.218.175.136]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C9A3035E944 for ; Fri, 14 Aug 2026 03:30:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.136 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786678242; cv=none; b=d+rLxtiviVmVP0zv+quAYL7mwuWitnkuHusdA7mF+5ArHHOPdNfPutwzQvwlraOg2ZG4FJq32L8QJgbVkuIH/bKpn3u+im4bbn9gu4TYGTmsHjD1gOe5x2GXCZl/r4iLgHcUfDSTBI01DvEJm1pFLfKfxmRG5LFMawLyCkQ1rX8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786678242; c=relaxed/simple; bh=GAsBrFVCIC/PHgQfkxgjYF3vPMw7obXhhWZeeIZbrC4=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=lifJEQl0hb7ds802eapQ7NgxgB1VtgCemn/53omgONZPbaHC33uDo1WZvYUpAwueWQJl4FFrcVGN6ty/od2jV6MnMagmNSBM8TOQjgpzc0zh5TLcm5XDR0cVksPPpjm2rUlsoUsJ+P0luspgvcBpHpcA40DhexNssNUUCKbGHao= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=PhFM+6dT; arc=none smtp.client-ip=91.218.175.136 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="PhFM+6dT" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=GAsBrFVCIC/PHgQfkxgjYF3vPMw7obXhhWZeeIZbrC4=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1786678238; v=1; x=1787283038; b=PhFM+6dT7p4lQwnUche71QzU72QF90Z8EGC4k55nIrPazKQd45F5/TSFULDUnnmedxlV702l LUriFoZ14qkQDYFqX9JTmJecKcp/x24mSK77QNSoz3NU0S3WJ0yZDLvnfut3B3rulCADXlbdTqs 3rwn6sV84OiCdgEO6xm7ogkM= X-Envelope-To: linux-kernel@vger.kernel.org Received: from mi-ThinkCentre-M760t.mioffice.cn (14.29.108.92) by smtp.migadu.com with ESMTPS id e5bbb15d4ae1c122; Fri, 14 Aug 2026 03:30:38 +0000 X-Migadu-Flow: FLOW_OUT From: Ridong Chen To: Johannes Weiner , Michal Hocko , Roman Gushchin , Shakeel Butt , Andrew Morton Cc: Muchun Song , David Finkel , Tejun Heo , cgroups@vger.kernel.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-mm@kvack.org (open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)), linux-kernel@vger.kernel.org, Tao Cui , Ridong Chen , Ridong Chen Subject: [PATCH v4 2/2] mm, memcg: fix memory.peak reset clobbering other fds' watermark Date: Fri, 14 Aug 2026 11:30:05 +0800 Message-Id: <20260814033005.2481920-3-ridong.chen@linux.dev> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20260814033005.2481920-1-ridong.chen@linux.dev> References: <20260814033005.2481920-1-ridong.chen@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Ridong Chen Writing to memory.peak resets the peak for that fd only. Each fd is a watcher and reads back max(its own value, the shared local_watermark). peak_write() resets by lowering local_watermark to the current usage. To keep the other watchers' peaks it then walks the watcher list, but it stores the current usage into them instead of the old watermark. So once usage has dropped from a peak, a reset on one fd wrongly drags every other fd's peak down too, even fds that never reset. Reproduced on 7.2.0-rc5-next under QEMU, two fds A and B on one cgroup: B sees the peak (410624 KB), usage drops, then A resets -- and B's peak collapses to 1060 KB although B never reset. With this patch B keeps reading 410624 KB. Fix: save the old watermark before lowering it and use that to floor the other watchers, so a reset only affects the fd that issued it. Fixes: c6f53ed8f213 ("mm, memcg: cg2 memory{.swap,}.peak write handlers") Closes: https://sashiko.dev/#/patchset/20260807090000.1532495-1-ridong.chen@linux.dev Assisted-by: Claude:claude-opus-4-8 Acked-by: Tao Cui Acked-by: Johannes Weiner Signed-off-by: Ridong Chen --- mm/memcontrol.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index ceb43b855122..5932046c637d 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -4744,7 +4744,7 @@ static ssize_t peak_write(struct kernfs_open_file *of, char *buf, size_t nbytes, loff_t off, struct page_counter *pc, struct list_head *watchers) { - unsigned long usage; + unsigned long usage, old_watermark; struct cgroup_of_peak *peer_ctx; struct mem_cgroup *memcg = mem_cgroup_from_css(of_css(of)); struct cgroup_of_peak *ofp = of_peak(of); @@ -4752,11 +4752,12 @@ static ssize_t peak_write(struct kernfs_open_file *of, char *buf, size_t nbytes, spin_lock(&memcg->peaks_lock); usage = page_counter_read(pc); + old_watermark = READ_ONCE(pc->local_watermark); WRITE_ONCE(pc->local_watermark, usage); list_for_each_entry(peer_ctx, watchers, list) - if (usage > peer_ctx->value) - WRITE_ONCE(peer_ctx->value, usage); + if (peer_ctx != ofp && old_watermark > peer_ctx->value) + WRITE_ONCE(peer_ctx->value, old_watermark); /* initial write, register watcher */ if (ofp->value == OFP_PEAK_UNSET) -- 2.34.1